In general, mobile communication systems have been developed to provide communication services to users in motion. Thanks to rapid technical advancement, mobile communication systems are capable of providing not only voice communication services but also high-speed data communication services.
Recently, 3GPP has been working to standardize specifications for the Long Term Evolution-Advanced (LTE-A) system, which is regarded as a next generation mobile communication system. The LTE-A system is expected to be standardized in late 2010, and aims to realize high-speed packet based communication supporting a data rate higher than existing ones.
As 3GPP standards evolve, schemes for efficient radio network optimization are proposed. Generally in initial radio network deployment or optimization, the operator of a base station or base station controller has to collect radio environment information related to cell coverage though drive tests. In existing drive tests, a measurer is inconvenienced by having to drive a car loaded with measurement instruments along the road for a long time in a repetitive way. Measurement results are analyzed and used to set system parameters for the base station or base station controller. Such drive tests increase optimization and operating costs of radio networks and are time consuming. As such, a study named MDT (Minimization of Drive Tests) is in progress to reduce manual drive tests and enhance the process of radio environment analysis and parameter setting. Instead of performing drive tests, a user equipment (UE) performs channel measurement and logs the channel measurement information. Then, the user equipment may immediately send the channel measurement information to the base station, send the same in response to occurrence of a particular event, or send the same after expiration of a preset time. Hereinafter, transmission of channel measurement information collected by the user equipment to the base station is referred to as channel measurement information reporting or MDT measurement information reporting. The user equipment may immediately send channel measurement results when it is communicable with the base station, or may save the channel measurement results and send the same later when it become communicable with the base station.
The base station uses the channel measurement information received from the user equipment to optimize cell coverage. In LTE-A, MDT measurement information reporting is basically classified by UE RRC state as follows.
TABLE 1UE RRC stateBasic MDT measurement information reporting of UEIdle modeLogging and deferred reportingConnected modeImmediate reportingConnected modeLogging and deferred reporting
MDT Measurement Information Reporting According to UE RRC State
Referring to Table 1, idle mode indicates a state in which the user equipment is not in communication with the base station, and connected mode indicates a state in which the user equipment is in communication with the base station. For MDT, as channel measurement information collected by the user equipment is transmitted through RRC signaling, the user equipment in idle mode is not required to transition to connected mode. In this case, the user equipment logs the collected channel measurement information and defers transmission of the same until transitioning to connected mode.
In connected mode, the user equipment may immediately send channel measurement information to the base station according to circumstances or may log the channel measurement information and send the same later as in idle mode.
In idle mode, the user equipment logs channel measurement information on an event basis or on a periodic basis. Examples of events triggering logging of channel measurement information are listed below.
(1) Periodic downlink pilot measurements
(2) Serving cell becomes worse than threshold
(3) Transmit power headroom becomes less than threshold
(4) Paging channel failure
(5) Broadcast channel failure
(6) Random access failure
(7) Radio link failure report
Logging events (1), (2), (4), (5) and (7) may be used to optimize downlink cell coverage, and logging events (3) and (6) may be used to optimize uplink cell coverage. The event (1) is related to periodic logging of channel measurement information, and the other events are related to logging of channel measurement information when a given condition is satisfied. The logged channel measurement information is reported to the base station when the user equipment transitions to connected mode. The user equipment may also log channel measurement information in connected mode.
The user equipment in connected mode sends logged channel measurement information to the base station when a reporting event is triggered. Currently, various report triggering conditions are considered in standard meetings. As channel measurement information for MDT is needed by the base station, a report request made by the base station (on demand) and a report trigger sent by the user equipment to the base station are considered as a report triggering condition.
As MDT procedures are not clearly determined, it is necessary to define an optimized MDT procedure.
For the purpose of MDT, an MDT procedure will be carried out during initial deployment and optimization of a radio network. That is, it is sufficient to conduct MDT only for a particular duration. Hence, it is necessary to notify the user equipment of the time to conduct MDT. In addition, it is unnecessary to handle the logging events proposed at standard meetings in all RRC states. For example, as it is possible to measure power headroom only in connected mode, the logging event (3) may occur only in connected mode. Hence, it is necessary to define a logging event on a mode basis in consideration of usefulness and ease of implementation. Transmission of channel measurement information from the user equipment to the base station may result in reduction of uplink capacity for other data transfer. Hence, to prevent waste of radio resources, it is preferable to allow the base station to selectively request only necessary information from the user equipment. In particular, a large amount of channel measurement information will be logged when the user equipment remains in idle mode for a long time. Upon transitioning to connected mode, the user equipment may consume a lot of resources to send the logged channel measurement information. Accordingly, MDT procedures are to be developed in consideration of the above problems.